1.0 Field of the Invention
The present invention relates to loudspeaker sometimes referred to as sound generators. More particularly, the present invention relates to a very low-mass, light-weight sound generator with a wide frequency bandwidth principally used in large surface area applications, such as wall covers, where mass is of crucial importance and when so used in such an arrangement is capable of delivering high sound levels required for audio generation or active sound control.
2.0 Description of the Related Art
A very wide variety of sound generators exist, the most familiar being the common loudspeaker. This and other such sound generators perform well in many applications, but all have disadvantages, which limit their range of applicability.
For example, conventional loudspeakers use high-mass voice coils. In aerospace applications where weight is a crucial expense, the use of loudspeakers can become prohibitive. Horn and buzzer type actuators can be designed which are light-weight and capable of low frequency use, however, their narrow-band nature and poor controllability limits their use to a narrow range of applications.
Polymer speakers have been successful in high frequency applications. These typically are electrostatic or piezoelectric (i.e. using poly-vinylidene fluoride film, abbreviated as PVDF). However, existing technologies are not capable of delivering the high displacement levels required for reproducing mid or low frequency audible sounds.
Aside from their use in sound generation, polymeric materials have been used in a bi-laminate configuration to generate motion. More particularly, when a voltage is applied to PVDF film (or any piezoelectric material) it changes thickness and length according to well-known constitutive piezoelectric equations. The thickness change is typically very small, but the length change can be significant. This elongation can be amplified by constructing a bi-laminar pair, often called a xe2x80x9cbimorph,xe2x80x9d which may be further described with reference to FIG. 1 showing a prior art parallel-laminate configuration 10.
FIG. 1 shows two layers 12 and 14 of PVDF film which are glued together with their polarities in the same direction in a manner known in the art. The voltage, xcex94V, to each PVDF film is applied between the center electrode (CE) (at the laminate interface) and the outer electrode (OE) of each laminate in a manner known in the art. FIG. 1 further illustrates each laminate as having an elongate length La, and a possible displacement y, whereas the combined thickness of the laminates, along with their associated electrodes is given by t.
The displacement xcex94y and force F generating ability of this laminate (and most simple actuators) is given by the usual expression.                               Δ          ⁢                      xe2x80x83                    ⁢          y                =                              (                          1              -                              F                                  F                  b                                                      )                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢                      y            0                                              (        1        )            
This equation contains two commonly measured parameters: the no-load tip displacement xcex94y0 and blocked-force Fb, defined as follows:                               Δ          ⁢                      xe2x80x83                    ⁢                      y            0                          =                              3            4                    ⁢                                    (                                                L                  a                                t                            )                        2                    ⁢                      d            31                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          V                                    (        2        )                                          F          b                =                              3            2                    ⁢                      t            ⁡                          (                              W                                  L                  a                                            )                                ⁢                      Yd            31                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          V                                    (        3        )            
With regard to expressions (2) and (3), t is the film thickness (of one layer, such as 12, of the bi-laminate made up of layers 12 and 14), La is the unconstrained length, W is the width of the parallel-laminate configuration 10, and xcex94V is the applied voltage. The parameters Y and d31 are respectively the Young""s modulus and the piezoelectric charge constant, both in the direction of length (the so-called xe2x80x9c31xe2x80x9d direction of the polymer). If multiple layer pairs, such as multiple pairs of layers 12 and 14, are used (in fully-bonded arrangements) the force increases by the square of the number of pairs.
Another common implementation is the series-laminate configuration (not shown), in which the polarities of the voltage potentials applied to the two layers, such as layers 12 and 14, are reversed and the positive voltage thereof is applied only across the outer two electrodes. This construction of the series-laminate configuration is simpler to fabricate (since it does not have a center electrode), but disadvantageously produces only half the deflection per applied volt.
The above bi-laminates, such as the parallel-laminate configuration 10 and the series-laminate configuration (not shown), is shown (e.g., FIG. 1) in the cantilever configuration, where one end is clamped and the other is free for movement thereof. An alternative configuration is called the xe2x80x9cbeamxe2x80x9d configuration, known in the art, in which both ends of the associated layers, such as layers 12 and 14, are clamped and the center of the associated layers is free to displace vertically.
An additional common configuration uses only one active layer, with the other layer being inactive. As used herein, an xe2x80x9cactivexe2x80x9d layer is meant to represent that the layer experiences movement and that the layer is comprised of an electro-acoustic material, such as a PVDF film. This one active layer arrangement is often called a xe2x80x9cmonomorph.xe2x80x9d It has reduced performance, but is of a lower cost.
The above bi-laminates have been previously used primarily as actuators for motion control. They have also found some use as sound generators in resonant (narrow bandwidth) alarm applications (typically using hard ceramic piezoelectric material) or for very low-level high-frequency novelty music sources. However, the prior art bi-laminate configurations have not used as broad-band sound generators. Therefore, a need exists in the prior art for bi-laminates that serve as broad-band sound generators.
An object of the present invention is to provide for bi-laminate configurations each having the ability to generate associated displacements so as to reproduce high sound levels required for audio generation or active sound control.
A further object of the present invention is to provide for various bi-laminates configurations, each of which serves as broad-band sound generators.
Another object of the present invention is to provide for bi-laminates that may be arranged into different configurations to provide for relatively large arrays all of which serve as broad-band sound generators.
Objects and advantages of the present invention are achieved by a bi-laminated members providing for an acoustic transducer. The acoustic transducer comprises a pair of bi-laminate members each having distal opposite ends. At least one layer of each of the pair of bi-laminate members being of an active electro-acoustic material. Each pair of bi-laminate members has inner and outer surfaces with a first electrode affixed to each outer surface of each pair of bi-laminate members and with a second electrode affixed to each inner surface of each pair of bi-laminate members. Each of the pair of the bi-laminate members extends along an elongated length and each of the pairs is affixed to one another at their respective distal opposite ends along the length. At least one of each of the pair of bi-laminate members has a curved central portion along the elongated length disposed between the distal opposite ends. The curved central portion of the bi-laminate member is displaced from its respective bi-laminate member in a direction transverse to the elongated length and effective so as to permit vibration of the bi-laminate members with respect to one another.